Adaptive channel estimation in a wireless communication system

ABSTRACT

A method and circuit for adaptively estimating channel conditions of a pilot channel in a wireless communication system. The method includes estimating channel statistics of the pilot channel, and adaptively filtering the pilot channel in response to the estimated channel statistics. The estimation is performed by filtering a channel signal derived from the pilot channel to determine an estimated channel mean and an estimated channel covariance. In order to perform the adaptive filtering, the present invention partitions the pilot channel into one or more time slots and weights each time slot according to the channel statistics. Thus, an advantage of the present invention is that it automatically and continually updates the pilot filter parameters in order to optimize the pilot filter performance over a broad range of channel conditions.

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention relates to wireless communication systems. Moreparticularly, the present invention relates to a novel and improvedmethod and apparatus for adaptively estimating the channel conditions ofa wireless communication channel.

II. Description of the Related Art

In a wireless radiotelephone communication system, many userscommunicate over a wireless channel. Communication over the wirelesschannel can be one of a variety of multiple access techniques that allowa large number of users in a limited frequency spectrum. These multipleaccess techniques include time division multiple access (TDMA),frequency division multiple access (FDMA), and code division multipleaccess (CDMA).

The CDMA technique has many advantages. An exemplary CDMA system isdescribed in U.S. Pat. No. 4,901,307, entitled “Spread Spectrum MultipleAccess Communication System Using Satellite Or Terrestrial Repeaters”,issued Feb. 13, 1990, assigned to the assignee of the present invention,and incorporated herein by reference. An exemplary CDMA system isfurther described in U.S. Pat. No. 5,103,459, entitled “System AndMethod For Generating Signal Waveforms In A CDMA Cellular TelephoneSystem”, issued Apr. 7, 1992, assigned to the assignee of the presentinvention, and incorporated herein by reference.

In each of the above patents, the use of a forward-link (base station tomobile station) pilot signal is disclosed. In a typical CDMA wirelesscommunication system, such as that described in EIA/TIA IS-95, the pilotsignal is a “beacon” transmitting a constant zero symbol and spread withthe same pseudonoise (PN) sequences used by the traffic bearing signals.The pilot signal is typically covered with the all-zero Walsh sequence.During initial system acquisition, the mobile station searches throughPN offsets to locate a base station's pilot signal. Once it has acquiredthe pilot signal, it can then derive a stable phase and magnitudereference for coherent demodulation, such as that described in U.S. Pat.No. 5,764,687 entitled “Mobile Demodulator Architecture For A SpreadSpectrum Multiple Access Communication System,” issued Jun. 9, 1998,assigned to the assignee of the present invention, and incorporatedherein by reference.

A functional block diagram of a typical prior art forward link dataformatter as used by a CDMA base station is shown in FIG. 1. Data source102 may be, for example, a variable rate vocoder such as that describedin U.S. Pat. No. 5,657,420, entitled “Variable Rate Vocoder”, issuedAug. 8, 1997, assigned to the assignee of the present invention andincorporated herein by reference. Data source 102 generates trafficchannel information in the form of frames of digital data. CRC and tailbit generator 104 calculates and appends cyclic redundancy check (CRC)bits and tail bits to the frames generated by data source 102. The frameis then provided to encoder 106, which provides forward error correctioncoding, such as convolutional encoding, upon the frame as is known inthe art. The encoded symbols are provided to repetition generator 120,which repeats the reordered symbols to provide the appropriatemodulation symbol rate. The repeated symbols are then provided tointerleaver 108, which re-orders the symbols in accordance with apredetermined interleaver format. The repeated, interleaved symbolstream is then covered with one of a set of orthogonal Walsh sequencesin traffic Walsh coverer 122, and gain adjusted in gain element 124. Itshould be understood that other forward link data formatters are alsoknown in the art. For example, it is well known that the repetitiongenerator 120 may be placed after the interleaver 108.

Pilot signal generator 128 generates a pilot signal, which may be asequence of all ones. The pilot signal is then covered with the all-oneWalsh sequence and combined with the output of gain element 124 incombiner 136. The combined pilot channel and traffic channel data (whichmay be plus or minus ones) is then spread in PN spreader 138 using acomplex PN code generated by PN generator 140, and then transmitted byradio frequency transmitter 142 over antenna 144. A similar forward linkdata formatter is disclosed in co-pending U.S. patent application Ser.No. 08/886,604, entitled “High Data Rate CDMA Wireless CommunicationSystem”, assigned to the assignee of the present invention andincorporated by reference herein.

Other data formatting techniques also exist. For example, in thecdma2000 reverse link, the pilot signal is time-multiplexed with powercontrol commands. Additionally, in W-CDMA, the forward link usesdedicated pilot signals that are time-multiplexed with otherinformation.

FIG. 2 illustrates a functional block diagram of a typical prior artdata demodulator for use in a CDMA mobile station. Receiver 202 receivesand downconverts the signals transmitted by transmitter 142 of FIG. 1.The digital baseband output of receiver 202 is despread in PN despreader204 using the complex PN code generated PN generator 206, which is thesame complex PN code as that generated by PN generator 140 of FIG. 1.

The despread signal is then Walsh uncovered in traffic channel Walshuncoverer 208 using the same Walsh sequence as that of the trafficchannel Walsh coverer 122 of FIG. 1. The Walsh-uncovered chips are thenaccumulated into Walsh symbols in Walsh chip summer 210 and provided asa traffic channel signal to dot product circuit 212. In someapplications, an additional delay element (not shown) is introducedbetween Walsh chip summer 210 and dot product circuit 212 to account fordelays introduced by pilot filter 216. However, if pilot filter 216 is acausal filter, such a delay element (not shown) is not necessary. Thedot product circuit is also known as a “conjugate product” circuit. Itperforms the operation expressed mathematically by one of the followingequivalent forms: <a,b>=a·b=ab*, where b* is the complex conjugate of b.

The despread signal is also provided to Walsh chip summer 214 where theyare accumulated into Walsh symbols and provided to pilot filter 216 aspilot channel symbols. Note that since the pilot channel is covered withthe all-one Walsh sequence in Walsh coverer 134 of FIG. 1, a vacuousoperation, the corresponding uncoverer is also vacuous in operation.However, in the general case, the pilot signal may be uncovered usingany same Walsh sequence as is used to cover it. The pilot filter 216serves to reject the noise in the pilot symbols, providing a phase andscale reference for the dot product circuit 212.

Once per symbol, the dot product circuit 212 computes the component ofthe traffic channel signal in phase with the pilot channel signalgenerated by the pilot filter 216. As described in U.S. Pat. No.5,506,865, entitled “Pilot Carrier Dot Product Circuit”, issued Apr. 9,1996, assigned to the assignee of the present invention and incorporatedherein by reference, the dot product adjusts both the received signal'sphase and scale as needed for coherent demodulation.

The symbols output from dot product circuit 212 are de-interleaved inde-interleaver 218, using the same format used by interleaver 108 ofFIG. 1. The de-interleaved symbols are then decoded in decoder 220according to the error correcting codes employed by encoder 106 of FIG.1. The resulting decoded symbols are analyzed on a frame-by-frame basisby CRC Check 222 to ensure that the frame was properly decoded. If theframe was properly decoded, then that decoded frame is forwarded forfurther processing. CRC Check 222 typically would examine the CRCportion of the frame, but may also use other frame quality indicationssuch as Yamamoto metrics.

A typical pilot filter 216 is implemented as an equal-weight finiteimpulse response (FIR) filter with all defining parameters (e.g.,weighting, window width, window center) remaining constant regardless ofthe channel conditions. Alternately, an exponential decay infiniteimpulse response (IIR) filter having fixed parameters (e.g., decay timeconstant, scaling) may be used. In other words, the designer of atypical prior art pilot filter 216 will choose a static filter designthat performs adequately for a given energy per bit to noise densityratio (E_(b)/N₀) under most channel conditions of interest, but notoptimally over the entire range of conditions.

As a mobile station (e.g., a cellular telephone, PCS telephone or otherwireless remote communication terminal) moves through the terrestrialenvironment, the signals it transmits and receives will experiencevarious types of fading. The mobile environment is usually characterizedby fading that can be either Rician or Rayleigh in nature. Other typesof fading are also possible. The fading characteristic in the typicalchannel signal is caused by the signal being reflected from manydifferent features of the physical environment, thus it is calledmultipath fading. At the UHF frequency bands usually employed for mobileradio communications, including those of cellular mobile telephonesystems, significant phase differences in signals traveling on differentpaths may occur. The possibility for both constructive and destructivesummation of the signals may result, with on occasion deep fadesoccurring.

Multipath channel fading is a function very sensitive to the physicalposition of the mobile unit. A small change in position of the mobileunit changes the physical delays of all the signal propagation paths,which further results in a different phase for each path. Thus, themotion of the mobile unit through the environment can result in a rapidfading process. For example, in the 850 MHz cellular radio frequencyband, this fading can typically be as fast as one fade per second forevery mile per hour of vehicle speed. Fading this severe can beextremely disruptive to signals in the terrestrial channel resulting inpoor communication quality, particularly as the speed of the mobilestation increases beyond 150 km/hr.

As previously stated, the typical fixed-parameter pilot filter 216 isnot optimized for such a broad range of channel conditions. It istypically designed to work adequately at speeds from stationary to about120 km/hr, or about as fast as a mobile station might be expected totravel in a motor vehicle on the highway. However, since the fadingcharacteristics of the channel are vastly different as between aslow-moving mobile station and a fast-moving mobile station, the typicalfixed-parameter pilot filter 216 cannot be optimized for both extremes.Typically, this forces the designer to design a pilot filter 216 thatworks well only when the mobile station is stationary or moves slowerthan about 150 km/hr, and works poorly beyond 150 km/hr. Astransportation such as bullet trains and airplanes exceeds this speed,it is unlikely that the user of a mobile station will be able to obtainreliable communications. Even when operational, the Signal-to-NoiseRatio (SNR) (or in other words, the E_(b)/N₀) of the communication linkmust be kept at a high enough level to be reliable in these severefading conditions. Increasing the E_(b)/N₀ of the communication linkdecreases the total capacity of the wireless system, particularly in aCDMA system where one transmitter's transmissions comprise interferenceto all other transmitters in the same CDMA frequency band. As a result,the designer of a fixed-parameter pilot filter 216 generally adopts anunfavorable compromise in selecting the filter parameters when facedwith such a broad range of channel conditions.

Thus, there is a need for a more optimal pilot filtering method andapparatus, particularly in the wireless communication environment, thatavoids these shortcomings in the prior art.

SUMMARY OF THE INVENTION

The present invention is a novel and improved method and circuit foradaptively estimating channel conditions of a pilot channel in awireless communication system. The method includes estimating channelstatistics of the pilot channel, and adaptively filtering the pilotchannel in response to the estimated channel statistics. The estimationis performed by filtering a channel signal derived from the pilotchannel, or any channel bearing ambiguous data with the ambiguityremoved after detection or decoding. This determines an estimatedchannel mean and an estimated channel covariance. In order to performthe adaptive filtering, the present invention partitions the pilotchannel into one or more time slots and weights each time slot accordingto the estimated channel statistics. Thus, an advantage of the presentinvention is that it automatically and continually updates the pilotfilter parameters in order to optimize the pilot filter performance overa broad range of channel conditions.

In one embodiment of the present invention, the channel signal isfiltered in one or more infinite impulse response (IIR) filters todetermine the channel statistics. In a another embodiment, the channelsignal is filtered in a combination of IIR and finite impulse response(FIR) filters to determine the channel statistics.

In an embodiment applicable to the cdma20000 system wherein the pilotchannel comprises a pilot signal portion having a known sign and a powercontrol bit portion having an unknown sign. The method of the presentinvention may also include determining the sign of the power control bitportion, correcting the sign of the power control bit portion, and thencombining the sign-corrected power control bit portion with the pilotsignal portion to generate the channel signal from which the channelstatistics are estimated by the channel statistics estimator. In yetanother embodiment, the method includes determining the sign of thetraffic signal and generating the channel signal in response to thetraffic signal.

In yet another embodiment applicable to a cdma2000 system, wherein thepilot channel comprises a sequence of power control groups, each powercontrol group having a pilot signal portion having a known sign and apower control bit portion having an unknown sign, the time slots have aduration substantially equal to the duration of the pilot signalportion.

A circuit for performing the method of the present invention is alsodescribed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, objects, and advantages of the present invention willbecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

FIG. 1 is functional block diagram of a typical prior art forward linkdata formatter as used by a CDMA base station;

FIG. 2 is a functional block diagram of a typical prior art demodulatorfor use in a CDMA mobile station;

FIG. 3 is a first embodiment of the present invention illustrated asbeing implemented in a cdma2000 reverse link traffic channeldemodulator; and

FIG. 4 is a second embodiment of the present invention illustrated asbeing implemented in a cdma2000 forward link power control bitdemodulator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described in an application that isspecific but not limited to the demodulation of a cdma2000 reverse linksignal. As is described in the proposed IS-2000 standard, the cdma2000reverse link signal comprises a traffic channel signal and a reverselink pilot signal. Punctured into the reverse link pilot channel(R-PICH) is a forward link power control subchannel that is a single bitthat spans the last quarter of each 1.25 ms power control group (PCG).This forward link power control bit is a signal from the mobile stationto the base station transmitter to either increase or decrease itstransmit power based on the mobile station's reception quality on theforward link. Such a reverse link pilot is described in detail inco-depending U.S. patent application Ser. No. 08/886,604,now U.S. Pat.No. 6,396,804, issued on May 28, 2002 to Odenwalder et al., entitled“HIGH DATA RATE CDMA WIRELESS COMMUNICATION SYSTEM”, assigned to theassignee of the present invention and incorporated by reference herein.

Because the R-PICH is used by the base station as a coherent referencefor data demodulation, a frequency reference for frequency tracking, anda received power reference for power control measurements, theuncertainty introduced by the forward link power control subchannel inthe otherwise determinate signal could degrade the reverse linkperformance. In other words, the puncturing of the forward link powercontrol subchannel into the R-PICH could lead to poorer the reverse linkperformance than a continuous pilot channel. For example, if we use onlythe non-punctured portion of the pilot channel to estimate the channel,the signal-to-noise ratio of the channel estimate calculated by the basestation is decreased. A technique for resolving the sign ambiguity ofthe R-PICH and reconstructing a continuous pilot signal therefrom isgiven in copending U.S. patent application Ser. No. 09/298,394, now U.S.Pat. No. 6,304,563 issued on Oct. 16, 2001 to Blessent et al. entitled“METHOD AND APPARATUS FOR PROCESSING A PUNCTURED PILOT CHANNEL”, filedJul. 12, 1999, assigned to the assignee of the present invention andincorporated herein by reference.

It will be understood by a person of ordinary skill in the art thatalthough the present invention is disclosed with reference to a cdma2000system having a R-PICH with a punctured power control subchannel, thepresent invention is equally applicable to other wireless communicationsystems having a non-punctured pilot channel. Thus, the followingfigures are intended to be example applications of the present inventionand are not intended to limit the present invention to the cdma2000system.

Turning now to FIG. 3, a first embodiment of the present invention isillustrated as being implemented in a cdma2000 reverse link trafficchannel demodulator. Channel statistic estimator 300 estimates channelstatistics which are used by adaptive non-causal channel estimator 302to adaptively filter the pilot channel as will be discussed in moredetail below. It should be noted that in other embodiments, indeed inthe embodiment of FIG. 4 described below, an adaptive causal channelestimator or filter may be used in place of adaptive non-causal channelestimator 302. However, in the embodiment of FIG. 3, a non-causal filteris used for channel estimation due to its improved performance over acausal filter when the delay of the non-causal filter is tolerable. Inthe exemplary embodiment of FIG. 3, adaptive non-causal channelestimator 302 is an FIR filter. However, in the general case, adaptivenon-causal channel estimator 302 may be an IIR filter or a hybrid filterwith both FIR and IIR characteristics. One simple example is a filterthat outputs the difference of two exponential decay IIR filters of thesame time constant but different weights, such that the effectiveimpulse response of the filter is of finite extent. Another examplewould be a cascaded FIR and IIR filter.

The filtered pilot channel symbols are then input to conjugate productcircuits 306 and 308. Conjugate product circuit 308 also receives as asecond input, the despread, Walsh uncovered, and delayed traffic channelthat has been delayed by delay circuit 310. Delay circuit 310 delays thetraffic channel by an amount substantially equal to the delay introducedby adaptive non-causal channel estimator 302. Conjugate product circuit308 produces traffic channel symbols (soft decisions) for furtherde-interleaving and decoding as is known in the art.

Conjugate product circuit 306 receives as a second input the delayedpilot channel that has been delayed by delay circuit 304. Delay circuit304 delays the pilot channel by an amount substantially equal to thedelay introduced by adaptive non-causal channel estimator 302. Conjugateproduct circuit 306 performs a conjugate multiplication of the estimator302 output with the delayed pilot channel which contains a power controlbit punctured into the reverse link pilot channel. Conjugate productcircuit then forwards the resultant power control bit signal to pastforward link (FL) power control bit (PCB) detector 314.

Past FL PCB detector 314 compares the resultant power control bit signalto a threshold and thereby determines the sign of the punctured FL PCB.It should be noted that the term “past” is used in describing theoperation performed by past FL PCB detector 314 because the powercontrol bit being detected at any given time is the power control bitfrom a past power control group, as delayed by delay 304 and adaptivenon-causal channel estimator 302. The FL PCB decision determined by pastFL PCB detector 314 is then forwarded to PCB sign corrector 316 wherethe sign of the PCB is flipped, if necessary, to match the sign of theremainder of the pilot channel (+1). The output of PCB sign corrector316 is then provided, along with the further delayed pilot channel fromdelay 312, to time multiplexer (MUX) 318. Time MUX 318 provides, duringthe first ¾ of the power control group, an output equal to the furtherdelayed pilot channel from delay 312, which compensates for theprocessing time of the past FL PCB detector 314. Time MUX 318 provides,during the remaining ¼ of the power control group, the output of PCBsign corrector 316 which is now equal in sign (+1) to the pilot channel.The resulting output signal from time MUX 318, is, therefore, areconstructed pilot channel signal of constant sign. A similar techniquefor continuous pilot channel reconstruction in given in theabove-incorporated co-pending application Ser. No. 09/298,394 now U.S.Pat. No. 6,304,563.

The reconstructed channel signal is provided to channel statisticestimator 300 which estimates channel statistics therefrom for use insetting the weighting factors used by adaptive non-causal channelestimator 302 as mentioned above. The generation of channel statisticsby channel statistic estimator 300 and the application of weightingfactors by adaptive non-causal channel estimator 302 will be discussedin further detail below.

It should be noted that the elements in the left-hand side of FIG. 3that are involved solely in generating the reconstructed channel signalfor use by channel statistic estimator 300 (i.e., delay 304, conjugateproduct circuit 306, delay 312, past FL PCB detector 314, PCB signcorrector 316 and time MUX 318) are necessary only for a punctured pilotchannel such as the R-PICH of the cdma2000 reverse link. These elementsmay be omitted in applications having a non-punctured or otherwisecontinuous pilot channel. In such a case, the pilot channel signalitself may be sufficient for use by channel statistic estimator 300.Furthermore, the channel signal may also be generated from theadditional energy contained in the traffic signal using the methoddescribed in co-pending U.S. patent application Ser. No. 09/289,073,entitled “CHANNEL ESTIMATION IN A CDMA WIRELESS COMMUNICATION SYSTEM”,filed Apr. 8, 1999, assigned to the assignee of the present inventionand incorporated herein by reference. In the just-mentioned patentapplication, the decoded traffic channel symbols are re-encoded,re-interleaved, weighted according to the frame quality indicator andcombined with the pilot channel estimate, which in FIG. 3, would be thereconstructed channel signal output from time MUX 318. Thus, in thegeneral case, the generation of the channel signal, i.e., a signal thatrepresents the channel, may be a weighted combination of contributionsfrom the pilot portion of the R-PICH, the sign-corrected FL PCB from theR-PICH, and the sign-corrected traffic from one or more trafficchannels.

Turning now to FIG. 4, a second embodiment of the present invention isillustrated as being implemented in a forward link power control bitdemodulator. In FIG. 4, channel statistic estimator 400, which may bethe same as channel statistic estimator 300 of FIG. 3, is illustrated asreceiving the channel signal, which may be the reconstructed channelsignal of FIG. 3 in the case of a the R-PICH demodulator of FIG. 3, or areconstructed channel signal from a sign-corrected traffic channel aftersuccessful decoding, re-encoding, and re-interleaving as detailed in theabove-referenced U.S. patent application Ser. No. 09/289,073, or aweighted combination of the two signals. In the general case, channelstatistics estimator 400 may be different than channel statisticsestimator 300. Both may use essentially the same algorithm, described indetail below, but they may operate on different data, or over differenttime slots, or even different channel signal inputs. The channel signalis operated on by channel statistic estimator 400 as will be describedfurther below in order to estimate the channel statistics by whichadaptive predictive causal channel estimator 402 will assign filterweighting coefficients. In the exemplary embodiment of FIG. 4, adaptivepredictive causal channel estimator 402 is an FIR filter. However, inthe general case, it may be an IIR filter, or a hybrid filter havingboth FIR and IIR characteristics.

Adaptive predictive causal channel estimator 402 provides a channelestimate through delay 404 to conjugate product circuit 406. Conjugateproduct circuit 406 performs a conjugate multiplication on the channelestimate and the pilot channel (in this case, the R-PICH) to demodulatethe FL PCB punctured into the R-PICH. The resultant power control bitsignal is then forwarded to current FL PCB detector 408 fordetermination of the sign of the current FL PCB. Current FL PCB detector408 may be of similar construction to past FL PCB detector 314 of FIG.3. Current FL PCB detector 408 estimates a current FL PCB decisionregarding the sign of the FL PCB by comparing the power control bitsignal from conjugate product circuit 406 to a threshold. The current FLPCB decision may then be used by the forward link transmitter (notshown) to increase or decrease its power as necessary.

As described above, in both the embodiments of FIG. 3 and FIG. 4, thechannel statistic estimators 300 and 400 estimate channel statistics foruse by adaptive non-causal channel estimator 302 and adaptive predictivecausal channel estimator 402, respectively, in setting their respectivefilter coefficients. To explain how this process occurs, the followingmathematical description will be introduced. First, consider thereceived channel signal of a particular Rake finger of a Rake receiverentering the mobile station channel statistic estimator 300 or 400input. This received signal is of the form:

y[n]=a[n]+w[n]  (1)

where

y[n] is a column vector representing the actual received channel signalat time slot n;

a[n] is a column vector representing the actual information signal as atrue representation of the channel at time slot n;

w[n] is a column vector representing the actual noise contained in thechannel signal at time slot n; and

n is a vector index in the time domain, and therefore may represent anarbitrary time slot for sampling the received channel signal.

Therefore, the dimension of the column vectors a[n], w[n], y[n], or thenumber of integral values that n can take on, is the same as the numberof time slots of received channel signal used as input for adaptivechannel estimation. In the second exemplary embodiment applicable to thepredictive estimation of a FL PCB in a cdma2000 R-PICH, each time slotmay be ¾ of a single 1.25 ms power control group (i.e., thenon-punctured portion of the pilot channel). Thus, if we use 2 PCG worthof channel signal as input for estimation, as n ranges from 1 and 2,y[1] represents the received channel signal of the current ¾ PCG ofpilot whereas y[2] represents the received channel signal of theprevious ¾ PCG of pilot. These y[1] and y[2] values can be obtained bysimple averaging of pilot symbols over the said ¾ PCG of pilots.

Other embodiments may use larger or smaller time slots depending on theresolution and estimation accuracy desired as a tradeoff to the costs ofcomputation. For example, in the cdma2000 embodiment, the punctured FLPCB may be one coherent time slot, with an ambiguous sign, or it cancomprise several smaller coherent time slots if finer resolution isdesired. In the general case, the time slots may be of unequal duration,though the received channel signal at each time slot could be weightedappropriately to normalize the information signal amplitude or,depending on the application, the signal-to-noise ratio. In otherembodiments, the time slots may be a configurable parameter. If apartition of the channel signal into finer time slots is used, the indexn can range from one to a higher number. If a non-causal estimator isdesired, as in the embodiment of FIG. 3, some of the elements of thecolumn vectors y[n], a[n], and w[n] may also represent values after thetime of interest in the system where the channel estimate is desired.

We must estimate the actual channel value x at a given moment, as shownbelow in Equation (2). Equation (2) represents the operation performedby both adaptive non-causal channel estimator 302 and adaptivepredictive causal channel estimator 402. One difference between the twoapplications are the different partitions of the channel signal intodifferent time slots as discussed above. Note that all estimates hereinare denoted by the {circumflex over ( )} (“hat”) notation Let x be thedesired channel value at a time slot of interest. For example, while theindex n for the y, a, and w vectors takes on the values 1 and 2representing the pilot portions of the pilot channel during the currentand previous PCGs as in the exemplary embodiment of FIG. 4, x representsthe channel value at the current PCB portion of the pilot channel,

{circumflex over (x)}={circumflex over (m)} _(x) +H(y[n]−{circumflexover (m)} _(y) [n])  (2)

where

{circumflex over (x)} is the estimated channel value at the time slotcorresponding to the PCB portion of the pilot channel,

{circumflex over (m)}_(x) is the estimated mean of x;

{circumflex over (m)}_(y)[n] is the estimated mean of the channel signalat time slot n; and

H is a filter vector described by H={circumflex over(K)}_(xy)({circumflex over (K)}_(yy))⁻¹

where:

{circumflex over (K)}_(xy) is the estimated covariance matrix of the xand y[n]; and

{circumflex over (K)}_(yy) is the estimated covariance matrix of thevector y[n], and the superscript −1 represents the matrix inversionoperation.

In other words, H is a vector representation of a linear filter thatwhen the filter is applied to an arbitrary signal z[n], the outputresulting from the operation can be described in the following in timedomain notation: $\begin{matrix}{{H\left( {z\lbrack k\rbrack} \right)} = {\sum\limits_{k}{{h\lbrack k\rbrack}\quad {z\lbrack k\rbrack}}}} & (3)\end{matrix}$

where k is the vector index.

It is also possible to make x into a vector by packing more values intoit when the channel estimate at multiple time instances are desired,especially if this provides extra implementational convenience in someembodiment of application due to the computational architecture used.Thus: $\begin{matrix}{{H\left( {z\lbrack k\rbrack} \right)} = {\sum\limits_{k}{{h\left\lbrack {j,k} \right\rbrack}\quad {z\lbrack k\rbrack}}}} & \text{(3a)}\end{matrix}$

where H now represents the filter matrix, and j and k are the vectorindices for the row and column, respectively, with j being the index ofa particular element {circumflex over (x)}[j] in the vector x to beestimated.

The values {circumflex over (m)}_(y)[n], {circumflex over (K)}_(xy), and{circumflex over (K)}_(yy) are what are referred to herein as theestimated channel statistics. These are the values estimated by channelstatistics estimator 300 and 400 for use by adaptive non-causal channelestimator 302 and adaptive predictive causal channel estimator 402,respectively, in determining the filter weighting used to estimate{circumflex over (x)}. Thus, these channel statistics are the neededcomponents of Equation (2) above, given that y[n], the actual receivedchannel signal, is already known.

Next, the generation of these channel statistics is described. Channelstatistic estimators 300 and 400 generate the estimated channel signalmean, {circumflex over (m)}_(y)[n], according to the followingrelationship:

{circumflex over (m)} _(y) [n]=g ₁ *y[n]  (4)

where g₁ is a filter impulse response, which in the general case may beeither an FIR or an IIR or hybrid filter that has both FIR and IIRcharacteristics; and the operator * represents convolution in the timedomain.

Note that y[n] now represents a signal vector in time, and the timeindex over which the convolution operation is applied is not shown. Ifit were shown, both {circumflex over (m)}_(y)[n] and y[n] would have twoindices, with the first one (shown) representing the time slots (e.g.“current PCG” and “previous PCG” as used for the second exemplaryembodiment of the application above), and the second (not shown)representing the updating of the vector values as what is “current time”proceeds.

The same method is used to calculate the estimate {circumflex over(m)}_(x)[n] since the elements of {circumflex over (m)}_(x)[n] representthe estimated channel just as the elements of {circumflex over(K)}_(y)[n] do, though at perhaps different time slots, so that only ashuffling of time slot indices is required before the direct applicationof the same method. Note that the channel mean usually does not changeoften, and when the time slots used as estimation inputs are closetogether in time, we may use one single value to represent all theelements in {circumflex over (m)}_(x)[n], {circumflex over (m)}_(y)[n]because they are similar in value, thus possibly saving computation. Inanother embodiment of application of the present invention in which thechannel is known to have zero-mean, {circumflex over (m)}_(x)[n],{circumflex over (m)}_(y)[n] can be taken to be zero, thus simplifyingEquation (2).

{circumflex over (x)}[n]=H(y[n])  (2b)

Channel statistic estimators 300 and 400 generate {circumflex over(K)}_(xy), the estimated covariance of x and the vector y[n] accordingto the following relationship:

{circumflex over (K)} _(xy) [m,n]=g ₂*(x[m]y*[n])  (5)

where

g₂ is a filter impulse response, which in the general case may be eitheran FIR or an IIR or hybrid filter that has both FIR and IIRcharacteristics;

m and n are the row and column vector indices, respectively; and

y*[n] is the conjugate of y[n].

Again, the time index over which the convolution operation is applied isnot shown.

Channel statistic estimators 300 and 400 generate {circumflex over(K)}_(yy), the estimated covariance of the y[n] vector according to thefollowing relationship:

{circumflex over (K)} _(yy) [n,m]=g ₃*(y[n]y*[m])  (6)

where

g₃ is a filter impulse response, which in the general case may be eitheran FIR or an IIR or hybrid filter that has both FIR and IIRcharacteristics; and

y*[m] is the conjugate of y[m].

Again, the time index over which the convolution operation is applied isnot shown. The present invention also contemplates taking advantage ofthe fact that {circumflex over (K)}_(yy) may be conjugate symmetric,that is,

{circumflex over (K)} _(yy) [m,n]={circumflex over (K)} _(yy)*[n,m]  (7)

such that only half of the values in the matrix {circumflex over(K)}_(yy) need to be computed.

In addition, if part of the received channel signal vector y[n] has timeslots of equal durations representing values equally spaced in time, wemay also take advantage of the shift-invariant property of thecovariance function and save computation further,

{circumflex over (K)} _(yy) [m,n]={circumflex over (K)} _(yy)[m+k,n+k]  (8)

where k is any integer value as long as m+k and n+k are within the scopeof valid indices of the matrix {circumflex over (K)}_(yy) and where theabove assumption about the partition of time slots is true.

The time constants of these three filters, g₁, g₂, g₃, are usuallypicked for the particular application at hand depending on the systemparameters. For example, if the channel statistics do not change over aduration of time on the order of one second, the time constants of thefilters g₁, g₂, g₃, may be chosen to be one second or shorter.

When multiple Rake fingers are used, a more sophisticated application ofthe present invention can make sure that the channel statistics from thedifferent fingers are the same or similar, and use all the channelsignals y[n] from different fingers for the statistics estimation,giving more accurate estimates due to the increased amount of availableinput for estimating the same or similar values of channel statistics.

So, as can be seen from the above mathematical relationships, channelstatistic estimators 300 and 400 generate the channel statistics{circumflex over (m)}_(x), {circumflex over (m)}_(y), {circumflex over(K)}_(xy), and {circumflex over (K)}_(yy). These are in turn provided tothe respective channel estimators (both adaptive non-causal channelestimator 302 and adaptive predictive causal channel estimator 402,which may also be referred to generically as “pilot filters”). Thechannel estimators of FIG. 3 and FIG. 4 then use these channelstatistics in carrying out the operation described by Equation (2)above, i.e., adaptively estimating the channel conditions.

Central to the present invention is the concept that the channelstatistics are slowly but constantly changing as the mobile stationmoves through the fading environment. These channel statistics arecontinually updated by channel statistic estimators 300 and 400 for useby adaptive non-causal channel estimator 302 and adaptive predictivecausal channel estimator 402, respectively. Because these channelstatistics form the basis for Equation (2), the adaptive filteringoperation, both adaptive non-causal channel estimator 302 and adaptivepredictive causal channel estimator 402 are changing their filterparameters in response to changes in channel conditions.

Advantageous to the present invention, Equation (2) gives an estimatorthat minimizes the mean squared error of the estimate output. Moreover,the present invention provides convenient and efficient methods toautomatically calculate all the ingredients for employing this equation.Thus, both adaptive non-causal channel estimator 302 and adaptivepredictive causal channel estimator 402 are continually updated bychannel statistic estimators 300 and 400, respectively, such that theyare optimized under the then-prevailing channel conditions. This allowsboth adaptive non-causal channel estimator 302 and adaptive predictivecausal channel estimator 402 to generate the best channel estimate for agiven channel condition under a broad range of channel conditions. Insharp contrast to the prior art, the present invention provides a methodand apparatus for adaptively estimating the channel conditions in awireless communication system based on the use of estimated channelstatistics to determine the channel estimator's filter parameters. Thus,the present invention is always optimized for the current channelconditions, resulting in a significantly lower communication linkE_(b)/N₀ requirement for a given bit error rate.

The previous description of the preferred embodiments is provided toenable any person skilled in the art to make or use the presentinvention. The various modifications to these embodiments will bereadily apparent to those skilled in the art, and the generic principlesdefined herein may be applied to other embodiments without the use ofthe inventive faculty. Thus, the present invention is not intended to belimited to the embodiments shown herein but is to be accorded the widestscope consistent with the principles and novel features disclosedherein.

I claim:
 1. A method for adaptively estimating channel conditions of apilot channel in a wireless communication system, the method comprisingthe steps of: estimating channel statistics of said pilot channel; andadaptively filtering said pilot channel in response to said estimatedchannel statistics; wherein said estimating step further comprisesfiltering a channel signal derived from any received communicationchannels to determine an estimated channel mean and an estimated channelcovariance; wherein said pilot channel comprises a pilot signal portionhaving a known sign and a power control bit portion having an unknownsign, the method further comprising the steps of: determining said signof said power control bit portion; correcting said sign of said powercontrol bit portion; combining said sign-corrected power control bitportion with said pilot signal portion to generate said channel signal.2. A method for adaptively estimating channel conditions of a pilotchannel in a wireless communication system, the method comprising thesteps of: estimating channel statistics of said pilot channel; andadaptively filtering said pilot channel in response to said estimatedchannel statistics; wherein said estimating step further comprisesfiltering a channel signal derived from any received communicationchannels to determine an estimated channel mean and an estimated channelcovariance; wherein said step of adaptively filtering said pilot channelcomprises the steps of: partitioning said pilot channel into one or moretime slots; and weighting each time slot according to said estimatedchannel statistics wherein said wireless communication system furthercomprises a traffic channel, said traffic channel having a trafficsignal of unknown signs the method further comprising the steps of:determining said signs of said traffic signal; generating said channelsignal in response to said traffic signal.
 3. A method for adaptivelyestimating channel conditions of a pilot channel in a wirelesscommunication system, the method comprising the steps of: estimatingchannel statistics of said pilot channel; and adaptively filtering saidpilot channel in response to said estimated channel statistics; whereinsaid estimating step further comprises filtering a channel signalderived from any received communication channels to determine anestimated channel mean and an estimated channel covariance; wherein saidstep of adaptively filtering said pilot channel comprises the steps of:partitioning said pilot channel into one or more time slots; andweighting each time slot according to said estimated channel statisticswherein said pilot channel comprises a sequence of power control groups,each power control group having a pilot signal portion having a knownsign and a power control bit portion having an unknown sign, and whereinsaid time slot has a duration substantially equal to the duration ofsaid pilot signal portion.
 4. A circuit for adaptively estimatingchannel conditions of a pilot channel in a wireless communicationsystem, the circuit comprising: a channel statistics estimator thatestimates channel statistics of said pilot channel; and an adaptivepilot filter that adaptively filters said pilot channel in response tosaid estimated channel statistics; wherein said channel statisticsestimator filters a channel signal derived from said pilot channel todetermine an estimated channel mean and an estimated channel covariance;wherein said pilot channel comprises a pilot signal portion having aknown sign and a power control bit portion having an unknown sign, thecircuit further comprising: a power control bit sign detector fordetermining said sign of said power control bit portion; a multiplexerfor combining said sign corrected power control bit portion with saidpilot signal portion to generate said channel signal.
 5. A circuit foradaptively estimating channel conditions of a pilot channel in awireless communication system, the circuit comprising: a channelstatistics estimator that estimates channel statistics of said pilotchannel; and an adaptive pilot filter that adaptively filters said pilotchannel in response to said estimated channel statistics; wherein saidchannel statistics estimator filters a channel signal derived from saidpilot channel to determine an estimated channel mean and an estimatedchannel covariance, wherein said channel statistics estimator partitionssaid pilot channel into one or more time slots, wherein said adaptivepilot filter weights each time slot according to said channelstatistics, wherein said pilot channel comprises a sequence of powercontrol groups, each power control group having a pilot signal portionhaving a known sign and a power control bit portion having an unknownsign, and wherein said time slot has a duration substantially equal tothe duration of said pilot signal portion.